Change color of labels that correspond to disabled inputs

- Add a new widget type for input and extend it from other input widgets
- Add a new label type that can be linked to an input widget
- Change the label color when the input's disabled state changes

OpenRA.Mods.Common/Widgets/SliderWidget.cs

@@ -16,9 +16,8 @@ using OpenRA.Widgets;
 
 namespace OpenRA.Mods.Common.Widgets
 {
-	public class SliderWidget : Widget
+	public class SliderWidget : InputWidget
 	{
-		public Func<bool> IsDisabled = () => false;
 		public event Action<float> OnChange = _ => { };
 		public int Ticks = 0;
 		public int TrackHeight = 5;